Conventionally, an optical element is known having at least on its one surface an optical functional section and a flange section on the periphery of the optical functional section. The optical functional section is molded by the optical functional section forming portion of a movable mold member. A portion corresponding to the optical functional section is ejected from a mold, to push out the optical element from the mold.
As a mold structure for such an optical element, a mold in which the mold surface (the optical functional section forming portion of a movable mold member) of a male mold corresponding to the optical functional section is formed on a movable mold is available. In that mold, when the mold is opened, the optical element is left on the movable mold as the molded product. Further, an ejecting mechanism for the optical element is formed on the movable mold. In the structure, the end face of the ejecting mechanism is the mold surface (the optical functional section forming portion of the movable mold member) corresponding to the surface of the optical functional section. When the optical element is to be released from the movable mold, the molded product is reliably pushed out through the surface of the optical functional section. Incidentally, a melted molding material flows through a pouring gate (sprue), sprue runner (runner), and then injection port (gate) into a molded product space (cavity) (for example, see Japanese Unexamined Patent Publication No. 2002-200652 as Patent reference 1 and Japanese Unexamined Patent Publication No. 2002-200654 as Patent reference 2).
An example of an optical element will be described in detail with reference to the accompanying drawings. FIGS. 8A to 8C are a front view, optical-axis sectional view, and enlarged optical-axis sectional view of the flange section, respectively, of a conventional optical element. Referring to FIGS. 8A to 8C, an optical element 70 is a positive lens as a molded product. The optical element 70 includes an object-side surface 71 having an optical functional section 71a with a fine structure 711a (see FIG. 8C), an image-side surface 72 having an aspherical optical functional section, and a flange section 73 having an inclined portion 73a on its periphery. The object-side surface 71 is formed with a movable mold member, and the image-side surface 72 is formed with a fixed mold member.
The conventional optical element, however, has the following problems.
(1) To form a molded product (optical element), a melted molding material flows through a pouring gate (sprue), sprue runner (runner), and then injection port (gate) into a molded product space (cavity). The material is cut at the gate to obtain a molded product. The mold is opened apart into a movable mold member and fixed mold member. Generally, the molded product (optical element) is left in the movable mold integrally with the sprue, runner, and gate. When the mold is to be opened, if a small gap is formed between the molded product and movable mold member, the formed optical functional section 71a shrinks thermally. With the thermally shrunk optical functional section being present, if the molded product is ejected through that mold surface (the optical functional section forming portion of the movable mold member) of the movable mold member which corresponds to the optical functional section, as the mold surface and the surface shape of the thermally shrunk optical functional section 71a do not coincide, the thermally shrunk optical functional section 71a is deformed by the mold surface. In particular, if the fine structure 711a including a plurality of ring-shape zones with fine steps is to be formed on the surface of the optical functional section 71a, the fine structure cannot be reproduced with high accuracy.
Factors that may hinder the fine structure 711a from being reproduced with high accuracy will be described with reference to the accompanying drawings. FIG. 9 is a partial sectional view showing the optical element 70 in a molded state. A male mold 622a of the movable mold member is in contact with the optical element 70 as a molded product through the fine structure 711a of the optical functional section 71a as a boundary.
FIGS. 10A to 10C and 11A to 11C are schematic views showing a forming process for the fine structure 711a shown in FIG. 9. FIGS. 10A to 10C show a case wherein the gap between the mold surface (the forming portion of the movable mold member which corresponds to the optical functional section) corresponding to the optical functional section 71a and the fine structure 711a is large, and FIGS. 11A to 11C show a case wherein the gap between the mold surface corresponding to the optical functional section 71a and the fine structure 711a is small.
The case wherein the gap between a mold surface 6221a and the fine structure 711a of the optical functional section 71a is large will be described with reference to FIGS. 10A to 10C. Assume that the molded product is left on the movable mold member side but a gap is formed between the mold surface 6221a and the fine structure 711a of the optical functional section 71a (see FIG. 10A). The molded product shrinks thermally by cooling in a direction perpendicular to the optical axis (see FIG. 10B). It is estimated that if the optical functional section 71a is ejected with the mold surface 6221a, as the mold surface 6221a and the surface shape of the thermally shrunk optical functional section do not coincide, the mold surface 6221a may deform (flatten the edge of) the thermally shrunk optical functional section 71a (see FIG. 10C).
The case wherein the gap between the mold surface 6221a and the fine structure 711a of the optical functional section 71a is small will be described with reference to FIGS. 11A to 11C. Assume that the molded product is left on the movable mold member side but a small gap is formed between the mold surface 6221a and the fine structure 711a of the optical functional section 71a (see FIG. 11A). The molded product shrinks thermally by cooling in a direction perpendicular to the optical axis (see FIG. 11B). It is estimated that if the optical functional section 71a is ejected with the mold surface 6221a, as the mold surface 6221a and the surface shape of the thermally shrunk optical functional section do not coincide, the optical functional section 71a may deform to a shape shown in FIG. 11B and a shape shown in FIG. 11C depending on the mold surface 6221a. 
Regardless of whether a gap is present between the mold surface 6221a corresponding to the optical functional section 71a and the fine structure 711a, the angle formed by the inclined portion 73a of the flange section 73 of the molded product and an optical axis X is large. Thus, when the optical element 70 thermally shrinks after molding, the inclined portion 73a shrinks without being inhibited by a mold surface K of the flange section of the male mold 622a. 
(2) The molded product is opened apart into the movable mold member and fixed mold member. Generally, the molded product (optical element) is left in the movable mold member integrally with the sprue, runner, and gate. Assume that a separation force with which that portion of the optical element 70 after the gate which is formed with the movable mold member is released from the movable mold member is determined as F1 and that a separation force with which that portion of the optical element 70 after the gate which is formed with the fixed mold member is released from the fixed mold member is determined as F2. F1 and F2 tend to vary in accordance with variations in molding temperature, molding pressure, and the like. Generally, although the molded product (optical element) is left in the movable mold member integrally with the sprue, runner, and gate, it is not necessarily uniformly left in the movable mold member, and is not always in tight contact with the movable mold member.